Field of the Invention
The invention concerns a joint for rigidly and fixedly connecting in end-to-end relationship consummable electrodes of graphite or carbon which are used in electric furnaces.
Electrodes of carbon or graphite for electric furnaces, in particular arc furnaces, are consumed in use thereof and they are replaced as they are consumed by connecting a fresh portion of electrode to the preceding portion. At their ends such electrode portions comprise internally screwthreaded sleeves or sockets into which is screwed a connector with a conical double screwthread or nipple also made of graphite or carbon. The empty space between the adjacent threads of the nipple and the sockets is generally filled with a cement which is preferably a good conductor of electric current. That assembly constitutes the electrode joint which is therefore required to provide an effective and permanent mechanical and electrical connection between each portion of electrode.
In fact in the course of use that joint is subjected to numerous mechanical and thermal stresses which can give rise to incidents which are highly prejudicial to good operation of the furnace such as loosening and even rupture of the screwed socket/nipple assembly. Thus the joint must be capable of withstanding the mechanical shocks and vibration caused by short-circuiting of the electric arc or simply by some handling operations in procedures for charging a furnace and casting metal, but also the thermal stresses which in the course of use result in differential expansion of the carbonaceous elements constituting the joint. Those stresses may be amplified by the occurrence at the location of the nipple/socket of zones involving poor contact or even no contact and where the level of resistivity increases, locally causing overheating of the joint due to the Joule effect.
Significant amounts of progress have been achieved with the use of a joint for electrodes which systematically comprises as a filling material a cement based on carbonaceous products, the physico-chemical characteristics of which are adapted to the thermal cycle to which the joint is subjected in order to maintain an effective and permanent mechanical and electrical connection between each electrode portion. As regards filling of the gap between the screwthreads of the nipple and the socket with cement, one of the procedures which is most frequently employed involves introducing the carbonaceous cement which is solid at ambient temperature into openings provided in the nipple, acting as reservoirs. The carbonaceous cement which is a bonding agent based in most cases on pitch is deposited in the reservoirs of the nipple before assembly to the electrode sockets. The joint which is formed in that way between two portions of electrode is rapidly heated as it moves towards the molten bath, which causes softening and then flow of the cement in the reservoir to the spaces which have remained free between the nipple/socket screwthreads, where the cement hardens before being coked as from 400.degree. C. and up to 800.degree. C. to form a deposit which adheres to the surfaces of the screwthreads, thus providing a permanent mechanical and electrical connection.
There are a number of different configurations of joints of the reservoir type, which have been the subject of numerous patents, in particular U.S. Pat. Nos 2,510,230, 2,828,162 and 3,419,296. As regards to the cements which are the most widely used with that type of joint, these are bonding compositions based on pitch, incorporating organic bonding agents which are cokable but viscous at low temperature such as tar, pitch material, synthetic resin in accordance with U.S. Pat. No. 3,055,789 (French No 1 230 258) or such as dextrin, a thermo-setting synthetic resin in accordance with U.S. Pat. No. 3,624,011 (French No 1 485 912). Those bonding compositions make it possible on the one hand to regularise the sequence of distribution of the plastic pitch, the viscosity of which can vary between 100.degree. C. and 200.degree. C. in very substantial proportions and on the other hand to promote by virtue of the presence of a thermosetting resin solidification of the cement as from 200.degree. or 300.degree. C. In actual fact the presence of a thermosetting material already disturbs the flow of the cement and therefore good distribution thereof in the temperature range provided for that purpose, that is to say 100.degree. to 200.degree. C.
That problem of distribution seems to have been resolved by the use of new pitch-based compositions incorporating an additive agent intended to promote expansion of the cement upon heating and therefore to facilitate distribution thereof in the thread gap as from 100.degree. C. Thus U.S. Pat. No. 3,976,496 (French No 2 204 673) recommends the use of a pasty cement of particulate pitch diluted in the bonding agent based on lignin sulphonate whose foaming properties after humidification and heating beyond 100.degree. C. are known. Likewise EP-A-0 260 529 describes an electrode joint in which the cement is formed for the major part by pitch with the addition of a minor amount of a foaming agent selected from the group consisting of sulphur, 2,4-dinitroaniline, and nitrated clarified oils. In the latter case the foaming agent affords the double advantage of promoting distribution of the cement between the screwthreads, by considerably reducing the temperature at which softening of the pitch begins, and significantly increasing the rate of coking of the pitch as from 350.degree. C.
On the other hand those types of joint are no longer suitable for electric furnace electrodes using the new procedures for protection from oxidation involving cooling the electrodes as much as possible, in particular by spraying water, and possibly depositing an anti-oxidising protective layer at the surface thereof by spraying an aqueous solution of a suitable salt, for example aluminium phosphate (EP-A-0 334 007) in the region between the electrode contact grippers and the closure of the furnace. In this case the heating cycle of the electrodes and consequently that of the joints is completely modified in such a way that the reservoir-type joints using the cements of the prior art as a filling material are no longer capable of correctly providing for an effective permanent mechanical and electrical connection between each electrode portion. More precisely, maintaining the electrodes for a prolonged period (from 5 to 10 hours) at temperatures of between 150.degree. and 250.degree. C., followed by a rapid rise to temperatures of higher than 500.degree. C. as the electrode passes through the closure of the furnace do not make it possible, with just pitch-based cements, to achieve sufficiently rapid hardening before 500.degree. C., so that the risks of loosening and an abnormal increase in the temperature of the joint become serious.
It has therefore been found necessary to develop a type of reservoir joint capable of providing a permanent and effect ire electrical and mechanical connection between electrodes which are cooled in use thereof and which are therefore subjected to a new thermal cycle, involving the use of a new cement which combines a large number of physico-chemical properties, namely:
stability in the solid state up to at least 60.degree. C. to be preserved in the reservoirs of the nipple, irrespective of the period prior to use and the climatic conditions, PA1 fusion at between 90.degree. C. and 120.degree. C. with a reduction in viscosity to less than 2500 cP followed by rapid solidification with hardening at between 120.degree. and 150.degree. C., and PA1 finally good thermal stability to at least 400.degree. C., being the beginning of coking, with the preservation of a fixed amount of carbon of at least 50%, ensuring a good mechanical and electrical connection for the junction, even after coking and up to 800.degree. C.